1. Field of Technology
The embodiments disclosed herein relate to switching power converters. More particularly, the embodiments disclosed herein relate to switching power converters for light-emitting diode (LED) drivers.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronic applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices including personal computers and high definition TVs, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
LEDs are current-driven devices, and thus regulating the current through the LEDs is an important control technique. A LED driver generally requires that a constant direct current (DC) current be provided to a LED load. Conventional techniques use primary feedback in a switching power converter to provide switching-cycle by switching-cycle output current regulation. The cycle-by-cycle constant current control generates an approximately constant power output since the LED load voltage is relatively constant.
However, LED drivers are required to provide high power factor to the input alternating current (AC) source. Power factor in switching power converters is defined as the ratio of the real power delivered to the load to the apparent power provided by the power source. Utility companies or government agencies require power factors in switching power converters to exceed a certain minimum level by regulation. Thus, switching power converters should deliver power from the power source to the load with a high power factor. Generally, high power factor requires that the input current follows the input voltage, such that a sinusoidal power flow results instead of a constant power flow which is converse to the approximately constant power output generated by cycle-by-cycle constant current control.
To provide high power factor, a controller of a conventional switching power converter uses primary feedback to sample the primary side current sense of the power converter using an analog-to-digital converter (ADC). The controller estimates the output current based on the primary side current sense. Based on the feedback of the primary side current sense, the conventional switching power converters can regulate average output current to provide high power factor. However, using the ADC in the conventional switching power converter increases system complexity. Furthermore, due to the high speed of the primary current of the conventional switching power converter, the ADC must be a high speed ADC in order to accurately sample the primary current thereby further increasing system costs.